1. Field of the Invention
The present invention generally relates to an electrolytic cell partitioned by an ion exchange membrane into an anode compartment and a cathode compartment and electrolytic process using the electrolytic cell. More specifically, it relates to an electrolytic cell in which a cathode compartment is separated by a non-perforated cathode plate into a gas generation room and a gas separation room and electrolytic process for carrying out electrolysis while removing a cathode gas generated on the cathode plate substantially perfectly by causing a circulating flow to occur between the gas generation room and the gas separation room by gas lift effect.
2. Description of Prior Art
Conventionally, in electrolysing an aqueous alkali metal chloride solution by an electrolytic cell providing an ion exchange membrane, a perforated cathode has been mostly employed. As the perforated cathode, typical are expanded metal sheets, punched metal sheets and metal screens.
When the electrolysis is effected using such perforated cathodes, a part of cathode gas generated on the cathode surface ascends between the ion exchange membrane and the cathode, and the other part of it passes through perforations to the backside of the cathode and ascends. However, residence of cathode gas generated on the cathode in the vicinity of the cathode, especially in a space between the cathode and the membrane, leads to an increase of electric resistance in liquid, thus resulting in increased cell voltage.
In an attempt to prevent the residence of gas bubbles in the vicinity of the cathode, Japanese Patent Non-examined Publication No. 114571/1977 discloses a process in which an expanded metal sheet is used as a cathode and the long axis of perforations is allowed to have a specific angle (0 to 45 degrees). Nonetheless, even the foregoing cathode fails to prevent the presence of cathode gas in a small gap between the cathode and the membrane, when the anode-cathode distance is not more than 2 mm, especially 1 mm, thereby resulting in an increase in cell voltage.
To solve the problem, Japanese Patent Non-examined Publication No. 56193/1978 discloses a process in which partition plates are located behind the cathode and the electrolysis is effected while allowing a circulating flow to occur naturally or forcibly. No. 46483/1978 discloses an electrolytic cell having the construction in which a catholyte inlet is provided at an upper portion of a cathode compartment and the downward flow is caused to take place by inertia of catholyte inlet flow in the cathode compartment, whereby the downward flow is introduced behind a separating barrier through the lower portion of the barrier while cathode gas generated on a perforated cathode are eliminated behind the cathode, and No. 81498/1978 proposes an electrolytic cell in which a louver type cathode comprising up-grade-bars extended toward a membrane is employed and catholyte is caused to flow from behind the cathode to thus form the circulating flow. However, even though these techniques are used, the removal of bubbles (gas) present in a small gap between the cathode and the membrane becomes insufficient when the anode-cathode distance is not more than about 2 mm, especially 1 mm, with a result that the circulating flow is introduced behind the perforated cathode to thereby result in an increase in the presence of bubbles between the membrane and the cathode, which shows a rising curve of cell voltage as indicated by FIG. 1.
For the purpose of overcoming these problems, Japanese Patent Non-examined Publication No. 23076/1982 teaches a process for preventing the adhesion of gas bubbles by providing an electro-conductive porous layer with gas and liquid permeability on the surface of the membrane. This process, however, is difficult to apply to an industrial scale electrolytic cell, namely, it is by no means easy to form the electro-conductive porous layer uniformly on the surface of a large membrane. Moreover, no result is expected as to the goal of rapidly removing gas present on the surface of the cathode. Furthermore, in Japanese Patent Non-examined Publication No. 116891/1981 it is proposed that the adhesion of gas is prevented by rendering the surface of the membrane rough, but the foregoing problems are not yet solved.